A Timelock pertains to a type of Smart Contract primitive that puts restrictions on the amount of Bitcoins until a specific time in the future or a block height.
Timelocks are commonplace in numerous Bitcoin smart contracts which includes payment channels and hashed timelock contracts. Timelocks may be used to lock-up Bitcoins being held as an investment, for a long time, say for months or years.
A Timelock is also used to render fee sniping less attractive by making it less profitable. It may also be used for trustless precomputed fee bumping.